1. Field of the Invention
The present invention generally relates to an automatic focus detecting device having a plurality of focus detecting areas and, more particularly, to the automatic focus detecting device which can be practiced in a single lens reflex camera or a video camera having an AF function of accomplishing an automatic focusing by discriminating an object to be photographed or videoed within the field of view.
2. Description of the Prior Art
Hitherto, an automatic focus adjusting device is widely used which is operable to move the photo-taking lens to an infocus position on the basis of a result of focus detection carried out by a focus detecting means. In such automatic focus adjusting device, when the photo-taking lens is moved to the infinity position, where the lens is focused on an object at infinity, it may happen that the focus detection of an object close to the camera cannot be achieved. In view of this, the use of a limit switch has been made to detect the arrival of the photo-taking lens at the infinity position or the closest position so that, in the event that the focus detection is disabled and it is detected the arrival of the lens at the infinity position or the closest position, the photo-taking lens can be moved in a reverse direction to search for the object which can be automatically focused. This is disclosed in, for example, the Japanese Laid-open Patent Publication No.58-224318 published in 1983.
Also, an attempt has been proposed in, for example, the Japanese Laid-open Patent Publication No. 59-48719 published in 1984 wherein, even though the photo-taking lens is not held at the infinity position or the closest position, in the event of the incapability of the focus detection, the photo-taking lens can be moved so that the direction of movement of the lens can be determined depending on the position at which the photo-taking lens is brought to a halt.
However, any one of these prior art devices is so designed that, in the automatic focus adjusting device having only one focus detecting area, the search is made to find the object the focus of which can be detected in the event that no focus detection is impossible.
Suppose the automatic focus detecting device having one focus detecting area at the center of the field of view and one focus detecting area on each side of the center of the field of view. In the AF camera utilizing such automatic focus detecting device having such plural focus detecting areas, it is a problem how the defocus amount for driving the lens from plural defocus amounts obtained from the plural focus detecting areas should be determined in order to focus the object desired to be photographed. If the object occupies a position spaced a short distance away from the focus detecting area at the center of the field of view, the focusing of such object may in a high probability meet the photographer's requirement.
However, it often occur that, depending on the type of and the focal length of the photo-taking lens, no focus detection is possible with respect to a main object occupying a position close to the camera. If objects located on respective sides of the center of the field of view are focused while this main object exists in the focus detecting area of the center of the field of view (this possibility is high) and where objects located at a position distant from the camera at which the focus detection is not impossible exist at the focus detecting areas on respective sides of the center of the field of view, this may be beyond the photographer's requirement.